1. Field of the Invention
This invention relates to a metallization method for the preparation of semiconductor devices. More particularly, it relates to a method enabling an aluminum (Al) based material to be uniformly buried in a connecting hole coated by a barrier metal layer inclusive of a layer of a titanium (Ti) based material.
2. Description of the Related Art
With increasingly refined design rule for semiconductor devices, as exemplified by VLSIs and ULSIs of recent origin, the diameter of the connecting hole bored in an interlayer insulating layer for establishing connection between upper metallization and lower metallization is also refined, with the aspect ratio occasionally exceeding unity. The upper metallization is usually formed by depositing an Al-based material by sputtering. However, sufficient step coverage can be achieved difficultly for burying the connecting hole having such a high aspect ratio, frequently leading to disconnections.
As a measure for improving step coverage, a high temperature bias sputtering method has recently been proposed. As introduced in Monthly Semiconductor World, published by Press Journal Co. Ltd., 1989, December issue, pages 186 to 188, and in IEEE/IRPS, 1989, pages 210 to 214, this technique resides in effecting sputtering by heating a wafer to hundreds of degrees Centigrade by means of a heater block and by applying an RF bias by means of the heater block. It is possible with this method to improve step coverage to produce an Al-based material layer having a planar surface by Al reflow effect and ion bombardment by application of the bias power. It has been reported in these treatises that, if a Ti layer is provided as a layer underlying the Al-based material layer, the Ti layer contributes to surface migration of Al atoms to achieve superior step coverage.
Needless to say, the Ti layer provided as a layer underlying the Al-based material layer is used in expectation of its function as a barrier metal layer. However, the Ti layer, while being a superior contact material in achieving a low resistance ohmic contact, cannot fulfil the function as a barrier metal layer satisfactorily. The reason is that, even if the Ti layer is provided alone between the Si substrate and the Al-based material layer, both the reaction between Si and Ti and the reaction between Ti and Al proceed simultaneously, so that Al spikes into the Si substrate cannot be prevented. For this reason, a TiN layer is superimposed on the Ti layer to provide a double-layer barrier metal system (Ti/TiN system). In recent years, a Ti/TiON double-layer barrier metal system is also proposed, in which O.sub.2 is introduced at the time of TiN growth to convert it to TiON. The purpose of the double-layer structure is to segregate oxygen in the TiN grain boundary for further improving the effect of preventing diffusion of Al in the grain boundary.
However, during high temperature sputtering, the Al-based material is in an intermediate state between the solid and the liquid and is highly sensitive to surface morphology or structure of the underlying layer. If such Ti/TiON based barrier metal layer is previously formed in the contact region, difficulties are presented in uniform burial of the connecting hole, even if it is attempted to deposit the Al-based material layer by high temperature bias sputtering.
For example, in the case of a substrate in which a Ti layer 2 and a TiON layer 3 are sequentially deposited as a barrier metal layer on an interlayer insulating film 1, an Al-based material 6, deposited on the substrate by high temperature bias sputtering, is not fluidized sufficiently on the TiON layer 3. The reason is that, as shown schematically in FIG. 1, the TiON layer 3 has a columnar crystal structure with a grain size of 5 to 20 nm and exhibits rough surface morphology or structure so that it is inferior in reactivity and wettability with respect to the Al-based material 6. The present inventors made attempts to lower the deposition rate to about one half the customary deposition rate, with a view to promoting the reaction in an interface between the TiON layer 3 and the Al-based material 6. However, it was found that the burial characteristics were not improved.
For overcoming such problem, the present inventors also made attempts to stack a Ti layer inherently superior in wettability and reactivity with respect to Al on the TiON layer 3 to provide a three-layered barrier metal layer 5 composed of the Ti layer 2, the TiON layer 3 and a Ti layer 4 as shown in FIG. 2. However, deposition of the Ti layer 4 merely resulted in accentuated surface roughnesses so that the Al-based material 6 could not be fluidized satisfactorily on the surface of the Ti layer 4.
FIGS. 3a and 3b illustrate what inconveniences are produced when the state explained with reference to FIGS. 1 and 2 is actually produced in the vicinity of a contact hole. In FIGS. 3a and 3b, parts or components similar to those shown in FIGS. 1 and 2 are indicated by the same reference numerals.
Referring to FIG. 3a, a wafer is shown in which an interlayer insulating film 11 having a contact hole 10 reaching an impurity diffusion layer 9 previously formed in a silicon substrate 8 is formed on the silicon substrate 8, and in which a Ti/TiON barrier metal layer 7 or a Ti/TiON/Ti barrier metal layer 5 is formed for covering the entire substrate surface.
However, since both the barrier metal layers 5, 7 exhibit rough surface morphology, the Al-based material 6, deposited thereon by high temperature bias sputtering, cannot be fluidized sufficiently in the vicinity of the opening end of the contact hole 10. If the Al-based material 6 continues to be deposited further in this state, voids 11 tend to be produced in the bulk material, even although the contact hole 10 appears to have been buried in the Al-based material 6. Besides, since a large number of nuclei about which the Al-base material 6 starts to be grown are generated at a time on the rough surface, the crystal grains of the Al-based material layer 6 tend to be refined to deteriorate resistance to electro-migration.